The present invention relates to an optical pick-up device by which recording and/or reproducing of the information is conducted, and to an optical element provided in the optical pick-up device.
Conventionally, as information recording media, there are CD using a light flux of wavelength of about 780 nm, DVD using a light flux of wavelength of 635-650 nm, and AOD (Advanced Optical Disc) using the light flux having wavelength of about 405 nm or blu-ray disc, and an optical pick-up device which can correspond also to any information recording medium such as AOD or blu-ray disc, DVD and/or CD is developed. On an optical functional surface of the optical element such as an objective lens provided in this optical pick-up device, a antireflective film to prevent the reflection of the light flux is provided.
Generally, the antireflective film has the reflection prevention function for each light flux of the wavelength of a broad band, and corresponding to the breadth of the wavelength region (hereinafter, it is denoted as the reflection prevention wavelength region) in which the reflection is to be prevented, or the magnitude of the refractive index of the optical element main body, the reflection factor for a target (hereinafter, it is denoted as a target reflection factor), it is structured by various layer numbers. Specifically, in the optical pick-up device which can correspond to AOD or blu-ray disc and DVD, for example, in the case where the reflection prevention wavelength region is 400-650 nm, the refractive index of the optical element main body is about 1.5-1.6, and the magnitude of the target reflection factor is not larger than 1%, the antireflective film of the optical element is generally structured by 5-7 layers. Further, in the optical pick-up device which can correspond to AOD or blu-ray disc, DVD and CD, when the reflection prevention wavelength region is 400-800 nm, the antireflective film is structured by 9 layers or more (for example, refer to Patent Document 1).
Hereupon, when the antireflective film number is increased, there is a problem that the production cost is increased, or water enters between layers and the spectral characteristic is changed. Further, particularly when the optical element main body is formed of optical plastic, there is problem that the environmental resistance is lowered such as a case where a crack is generated on the antireflective film by the stress of the antireflective film itself, or the adhesion between the antireflective film and optical element main body is lowered.
Therefore, in order to solve such a problem, there is a technology by which, by limiting the wavelength whose reflection factor is not larger than 1% to only two kinds of wavelengths, the antireflective film is structured by few layer numbers of 3-7 (for example, refer to Patent Document 2).
(Patent Document 1)                Tokkai 2000-111702        
(Patent Document 2)                Tokkaihei 11-167003        
However, the technology disclosed in the above Patent Document 2 is a technology by which the reflection of the light flux having wavelength 150-300 nm or the light flux having wavelength 400-800 nm is prevented, and is not a technology by which the reflection of the light flux of wavelength 390-430 nm or the light flux of wavelength of 630-800 nm is prevented.
The object of the present invention is to provide an optical element by which the reflection of the light flux of wavelength of 390-430 nm or the light flux of wavelength of 630-800 nm can be prevented using the antireflective film with fewer layer numbers than conventional devices, and an optical pick-up device provided with this optical element.
A structure written in item 1 is an optical element which is provided in an optical pick-up device which conducts the recording and/or reproducing of the information, and which light-converges light fluxes of a plurality of wavelengths including wavelength λ1 (390 nm≦λ1≦430 nm) and wavelength λ2 (630≦λ2≦800 nm) on the information recording medium, and there are provided with one or more optical element main bodies, and the antireflective film which is provided on the surface of the optical element main body and on which at least one optical functional surface is formed, and the reflection factor of the light flux incident perpendicularly on the optical functional surface shows the local maximal value not smaller than 1% between the wavelength λ1 and the wavelength λ2. Furthermore, it is preferable that the local maximal value is not smaller than 2%.
Herein, the plurality of wavelengths means the wavelengths of 2 kinds or more. Further, “between the wavelength λ1 and the wavelength λ2” means a range which is longer than the wavelength λ1, and shorter than the wavelength λ2.
According to the above-described structure, because the reflection factor of the light flux incident perpendicularly on the optical functional surface shows the local maximal value not smaller than 1% between the wavelength λ1 and the wavelength λ2, the reflection factor is relatively low to the light flux of wavelength λ1 and the light flux of wavelength λ2, and is in the situation in which the reflection is prevented. Accordingly, by reducing the reflection prevention function between the wavelength λ1 and the wavelength λ2, the layer numbers of the antireflective film can be reduced without losing the reflection prevention function to the light flux of wavelength λ1 and the light flux of wavelength λ2 against the conventional antireflective film by which the reflection is prevented to the whole of light fluxes of the broad wavelength region of wavelength λ1-λ2. Accordingly, the production cost can be reduced, and a change of the spectral characteristic by entering of water between layers of the antireflective film can be suppressed.
Further, even when the optical element main body is made of plastic, a case where a crack is generated in the antireflective film by the stress of the antireflective film, or the adherence of the antireflective film and the optical element main body is lowered, can be prevented, that is, the environmental resistance can be improved.